In the operation of commercial aircraft, it is necessary to provide on-board toilet facilities. These toilet facilities include at least one holding tank to store the liquid and solid waste resulting from toilet use by passengers together with the liquid used to provide flushing action. Although the waste includes both liquid and solid materials, the content of the holding tanks will be referred to hereinafter collectively as liquid waste. In some systems, called recirculating systems, a chemical disinfectant fluid is recirculated from the holding tank through the toilet in successive flushing cycles. The discussion which follows will primarily be concerned with liquid waste from recirculating toilet tank systems using chemical disinfectant liquids. However, in other, non-recirculating systems, fresh water may be used in single flush operations, and the wastes are collected in a holding tank.
When an aircraft lands, the liquid waste must be emptied from the holding tank. Typically, each holding tank has an outlet tube extending from an opening in the bottom surface of the tank downwardly and outwardly to an access port mounted in the skin of the fuselage. Flow of the liquid waste out of the tank into the outlet tube is controlled by a discharge valve operated by a valve actuator. The valve actuator moves the discharge valve between a seated position, closing passage through the outlet tube, and an unseated position in which fluid can flow through the outlet tube. A serious problem with such systems is the possibility of leakage of liquid waste past an inadequately sealing discharge valve into the outlet tube during flight. To minimize the risk associated with this occurrence, it is common to require a separate removable plug at the outer end of the outlet tube, which provides a secondary closure. An access flap hinged or removably connected to the aircraft skin at the access port provides a third level of closure. Even despite the second and third levels of closure, unfortunate occurrences still happen in which significant volumes of liquid waste escape outside of the aircraft during flight.
This phenomenon has been common enough to be known in the airline industry as the build-up of "blue ice," so called because of the blue color of chemical disinfectants commonly used in aircraft toilets. Because of the cold temperatures at altitude, liquid wastes escaping past the tank discharge valve can sometimes leak around the access door to the exterior freeze and build up accretions of frozen blue ice. Losses of this blue ice may occur as the aircraft descends to warmer altitudes for landing which causes partial thawing and detachment of the blue ice accretions. There are periodic reported instances of frozen toilet wastes landing in the backyards of homes near airports or crashing through their roofs. Such an incident is reported, for example, in the newspaper "USA Today" for Oct. 21, 1992 at page 3A under the headline "The sky's not falling, that's passenger poop." The dangerous and highly distasteful consequences of blue ice incidents are not confined to those on the ground. The detached blue ice can be deflected into the intakes of jet engines on the aircraft causing costly engine corrosion, game outs and even life-threatening situations.
Accordingly, it has been an important aim of the designers of aircraft waste holding systems to devise a tank discharge valve and actuator which will minimize the risk of waste leakage through the outlet tube during flight. One such prior system is disclosed, for example, in U.S. Pat. No. Re. 32,750 to Grills and Colditz for "Fluid Operated Waste Tank Servicing Assembly." Such a system uses a holding tank having an outlet tube in its lower wall with a chamfered shoulder at its opening to the tank. A cylindrical valve, of larger diameter than the outlet tube, has a chamfered resilient seal which engages the chamfered shoulder to seal against it. The valve is raised and lowered from its seated condition on the shoulder by an actuator shaft connected, by a piston and cylinder arrangement, to the upper wall of the holding tank.
While systems of this general type may be generally satisfactory for their intended purposes, leakage problems can arise in connection with a plunger which relies upon seating on top of a chamfered or rounded shoulder at the junction between the lower wall of the holding tank and the outlet tube. For example, aircraft passengers sometimes gush items such as Q-tips, tampons, hairpins, razor blades, bottle tops, etc. down the toilet. During the emptying of the tank at the end of the flight, these articles can become lodged on the shoulder of the outlet opening while the valve is raised and can then prevent it closing properly which creates a leakage condition for the next flight. Over an extended period of use, attempted closures of the valve on top of obstructing foreign objects can cause surface blemishes in the shoulder, such as scratches or indentations, which create seepage channels through which liquid waste can bypass the valve even when no obstruction is present.
An alternative approach is to utilize a valve which does not rely on a seal between an oversized valve and a shoulder or chamfer at the intersection between the bottom of the tank and the outlet tube but, instead, uses a valve which is inserted into the inside of the outlet tube and incorporates a sidewardly facing peripheral seal. Such a sealing arrangement is shown, for example, in U.S. Pat. No. 4,069,952 to Martin for "Containers For Noxious Substances." The Martin patent is concerned with a container for emptying the contents of chemical toilets but does not specifically address the aircraft environment. With such an arrangement, it is important to ensure that the side seal on the valve has penetrated sufficiently far down into the outlet tube to have moved past any surface blemishes in the shoulder or chamfer area and to have wiped away any obstructions adhered to the sidewalls of the outlet tube that could interfere with sealing performance. From the point of view of the pilot of an airliner, he could have no assurance that this condition had been achieved with a device such as that disclosed in the Martin patent since it was not addressed to the specific problems of aircraft waste systems. In particular, the Martin patent did not suggest a warning system for indication of an inadequate penetration of the discharge valve into the outlet tube to ensure satisfactory sealing between the valve seal and the sides of the outlet tube.
In sum, the continuance of blue ice incidents, as periodically reported in the press, shows that the problem of leakage past the tank discharge valve persists despite the devices of the prior art.